Method for heating fresh water for domestic or industrial use

ABSTRACT

A method and apparatus for heating service water and providing heat for the heating of buildings, if required, including a first heat source, a storage tank, and a service water heat exchanger for heating service water, where a simple, robust and efficient system is attained by connecting the first channel of the service water heat exchanger in parallel with the storage tank and by providing a control device, which controls the first circulator pump and the second circulator pump in such a way that in a first operational state the mass flow of the heat transport medium through the first heat source is greater than the mass flow of the heat transport medium through the first channel of the service water heat exchanger, and that in a second operational state the mass flow of the heat transport medium through the first heat source is smaller than the mass flow of the heat transport medium through the first channel of the service water heat exchanger

TECHNICAL FIELD

The invention relates to a method for heating fresh water for domestic or industrial use and for providing heat for the heating of buildings, having a first heat source, a storage tank and a fresh water heat exchanger for heating fresh water, where the flow of a heat transport medium through the first heat source is effected by a first circulator pump and the flow of the heat transport medium through a first channel of the fresh water heat exchanger is effected by a second circulator pump.

The invention specifically relates to a method with a first heat source, a storage tank and a fresh water heat exchanger for heating fresh water, where the flow of a heat transport medium through the first heat source is effected by a first circulator pump and the flow of the heat transport medium through a first channel of the fresh water heat exchanger is effected by a second circulator pump.

BACKGROUND

When alternative energy sources, for instance solar energy, are employed for heating fresh water, a heat source must be provided which will supply the required amount if alternative energy is not available in sufficient quantity. Frequently adopted solutions provide a storage tank whose content is heated by the alternative energy, if available. In these solutions fresh water can be heated in two different ways. On the one hand it is possible to withdraw heat transport medium from the storage tank and to heat fresh water via a heat exchanger through which this heat transport medium flows. In this case supplying fresh water of constant temperature is difficult as regards control technology. As an alternative fresh water may be heated in a pipe coil inside the storage tank, which will partially avoid some of the indicated problems. In this case it will be difficult, however, above all with hot water circulation systems, to maintain a layered temperature in the storage tank in all operational states. This will reduce efficiency, especially in solar systems. A further disadvantage of this solution is that a powerful conventional heat source must be provided for compact systems with relatively small tank volume if a drop in fresh water temperature at peak loads is to be avoided.

It is furthermore known to provide condensing boilers with a fresh water tank to cope with peak loads. This fresh water tank must be enamelled or made of stainless steel to avoid contamination of the fresh water. Such a system is costly and also undesirable from a health perspective since it is subject to the danger of legionella growth.

The standard type of a solar thermal system has a heat transport medium flowing through the solar collector in a closed circuit. Since the solar collector always remains completely filled, protective measures have to be taken to ensure safe operation, i.e., expansion tanks, antifreeze additives etc. must be provided. In certain cases a simplified design is desirable to obtain particular robustness and cost-efficiency. To this end solar thermal systems have been developed in which the solar collector is filled with the heat transport medium only during operation and is emptied otherwise. Such a system is laid open in DE 20 20 6564 U. The heat transport medium is taken from a storage tank and is pumped through the solar collector to be heated when solar energy is available. When the pump is deactivated the solar collector is drained, eliminating the danger of freezing and making an antifreeze agent unnecessary. If the storage tank is already fully loaded, overheating or vaporization of the fluid can be avoided, eliminating the dangers of stagnation.

It is a disadvantage of the above solution that at least one heat exchanger is required if the heat is to be used for heating a building, and that strong insolation and little water consumption may lead to losses of the heat transport medium due to evaporation. In addition, atmospheric oxygen may cause corrosion.

U.S. Pat. No. 4,269,167 A describes a closed system comprising solar collector, expansion tank and heat exchanger. By pressurizing the system evaporation and oxygen problems may be reduced but the disadvantages of low efficiency and high cost will remain.

It is therefore desirable to avoid the above disadvantages and to propose a solution which is simple, economical, robust and efficient at the same time. Robustness is not only meant in a mechanical sense but also includes problem-free control behavior. By “efficient” is meant high thermal efficiency and good utilization of the energy available.

BRIEF SUMMARY

The invention seeks to avoid these disadvantages and to propose a method which permits optimum use of available alternative energy and at the same time requires only a conventional heat source of relatively low power while ensuring sufficient supply of hot fresh water.

The invention further seeks to propose an apparatus for the heating of fresh water, which has a simple design and is suited for implementing the method described above.

According to the invention the first channel of the fresh water heat exchanger is connected in parallel with the storage tank and a control unit is provided, which controls the first circulator pump and the second circulator pump in such a way that in a first operational state the mass flow of the heat transport medium through the first heat source is greater than the mass flow of the heat transport medium through the first channel of the fresh water heat exchanger, and that in a second operational state the mass flow of the heat transport medium through the first heat source is smaller than the mass flow of the heat transport medium through the first channel of the fresh water heat exchanger. It is essential for the method of the invention, that a fresh-water system is intended, i.e., that fresh water is fed to the consumer immediately after heating and is not intermediately stored in the heated state. The danger of legionella growth is thus avoided.

A further essential aspect of the invention lies in the fact that, as seen from the first heat source, the first channel of the fresh water heat exchanger is connected parallel to the storage tank. In the first operational state the demand for heat at the fresh water heat exchanger is small, for instance if only a small amount of hot water is withdrawn. Due to the prevailing temperature situation the heat cannot be provided by the storage tank, therefore the first heat source is activated. The first circulator pump circulates heat transport medium through the first heat source, such that the desired output temperature is achieved on the downstream side. Simultaneously the second circulator pump is controlled in such a way that the fresh water temperature is precisely kept at the desired level. Since in this operational state the heat required at the fresh water heat exchanger is less than the power offered by the first heat source, only part of the heat transport medium from the first heat source is fed to the fresh water heat exchanger. The remaining part flows through the storage tank from top to bottom, heating the tank content while a prevailing temperature stratification is only minimally disturbed.

In contrast to the situation described above the heat required at the fresh water heat exchanger in the second operational state is greater than the maximum power which the first heat source can supply. This can be the case if water is drawn at more than one tap or if a bath tub is to be filled. In this operational state the second circulator pump is operated at a high throughput rate, in order to supply the required thermal power at the fresh water heat exchanger. The flow rate through the first channel of the fresh water heat exchanger is now greater than that through the first heat source. This will lead to an inversion of the flow direction in the high-temperature line between the fresh water heat exchanger and the storage tank. The second circulator pump now sucks hot heat transport medium from the upper section of the storage tank and pumps it through the first channel of the fresh water heat exchanger together with the heat transport medium that has been heated by the first heat source. In this way peak loads of fresh water withdrawal can easily be coped with.

A third operational state, which could also be seen as a special case of the second operational state, occurs when the storage tank contains enough hot heat transport medium to feed the fresh water heat exchanger. In this case the first heat source is not active and the fresh water heat exchanger is fed with heat transport medium from the storage tank exclusively by the second circulator pump. Since obviously the first circulator pump is not active, the criterium is again met that the mass flow of heat transport medium through the first channel of the fresh water heat exchanger is greater than the mass flow through the first heat source, which in this case is zero. A decision to switch the heat source on or off is taken depending on the signal of a temperature sensor, which could be placed in the upper fourth of the storage tank. The storage volume above the temperature sensor thus defines the minimum amount of hot heat transport medium which is permanently available.

An essential advantage of the method proposed by the invention lies in the optimized control regime, since the double feeding of the fresh water heat exchanger will permit a very fast response to peak loads. Furthermore a system implementing the method of the invention will only need a relatively small, low-power device as a conventional heat source. Above all it will not be necessary to provide for modulated operation of the first heat source, which will offer a welcome simplification.

It is of particular advantage to let a partial stream of the heat transport medium be cycled by the second circulator pump via a mixing valve and a mixing line, when a predefined temperature threshold is exceeded in the fresh water heat exchanger. For optimum use of the storage tank it is desirable to set the maximum temperature of the heat transport medium at as high a value as possible. Without cycling, the fresh water heat exchanger would receive heat transport medium at a temperature of 90° C. or more, which would cause increased scaling. The first mixing valve can reduce the thermal load on the first channel of the fresh water heat exchanger, thereby reducing the tendency to calcification.

The heat which is available at a lower temperature level in the storage tank may additionally be used to preheat the fresh water in a heat exchanger inside the storage tank. Ideally this preheating will produce a water temperature whose level will equal or even exceed the target temperature. In this case the second circulator pump need not be activated, the fresh water flowing through the fresh water heat exchanger without being further heated. If the temperature of the fresh water is too high, cold water is added via a second mixing valve to obtain the target temperature.

From the point of view of overall efficiency it will be especially advantageous to maintain temperature stratification in the storage tank and to feed the heat transport medium that has been heated by the first heat source into an upper section of the storage tank in a first operational state, while in a second operational state the heat transport medium exiting from the first channel of the fresh water heat exchanger is fed into a lower section of the storage tank. The lower section mentioned need not necessarily be at the lowest point, since below the storage tank section for the fresh water cycle a further section may hold the heat exchanger for preheating the fresh water.

It is particularly advantageous if the first and the second circulator pumps can be controlled in a continuous manner, permitting continuous variation of the flow rate in the respective line sections. Controlled-rpm pumps can be used for this purpose. In this way the target temperature of the fresh water can be set and maintained most precisely.

A particular simplification of the method of the invention is attained if the heat transport medium of the fresh water system is directly fed into the heating system of a building. In this way it is possible to avoid the heat transfer via a heat exchanger, making better use of the available energy since the otherwise unavoidable temperature drop of 3 K to 5 K in the heat exchanger is eliminated. An especially preferred variant of this solution provides that the heat transport medium for a first heating system is taken from a high-temperature line connecting the downstream side of the first heat source to the storage tank, while the heat transport medium for a second heating system is taken from a low-temperature line connecting the upstream side of the first heat source to the storage tank. The first heating system typically is a radiator circuit requiring higher temperatures. The second heating system is for instance a floor or wall heating system, for which lower temperatures are sufficient.

Advantageously, temperature stratification in the storage tank can be maintained by introducing the back-flow of at least one heating system in the storage tank at a point below the fittings for the fresh water heating system.

A further preferred variant of the invention proposes that a solar collector be filled with the through-flowing heat transport medium when insolation is present for heating the medium and that the solar collector be drained otherwise, and that the heat transport medium be collected in a storage tank and held under increased pressure in the storage tank and in the solar collector, and that the storage tank in all operational states be filled partly with the heat transport medium and partly with gas. It is essential that the whole system is permanently held under pressure, which fact permits the simultaneous achieving of a plurality of objectives.

The pressurized heat transport medium can be directly used for heating a building, making any sort of heat exchanger in the heating system unnecessary. A heat exchanger typically causes a temperature loss of 3 K to 5 K, with a corresponding loss of efficiency. At the same time the solution according to the invention eliminates the necessity of conventional expansion tanks, since the storage tank itself acts as expansion tank. Since the boiling point of pressurized heat transport medium is higher it can be heated to higher temperatures in the solar collector. Typically the heat transport medium is mainly water, which boils at 100° C. under normal pressure. The solution of the invention permits heating up to 120° C. to 140° C. As the system is closed losses of the heat transport medium are avoided.

In the simplest case air is used as process gas, but systems filled with nitrogen would also make sense. The pressure of the system is typically set to a value between 2 bar and 5 bar.

Furthermore the present invention relates to an apparatus for heating fresh water and possibly for the supply of heat for heating buildings, having a first heat source and a storage tank and a fresh water heat exchanger for heating fresh water, where the downstream side of the first heat source is connected to the storage tank via a high-temperature line and the upstream side of the first heat source is connected to the storage tank via a low-temperature line, and further having a first circulator pump which generates flow through the first heat source.

According to the invention this apparatus is characterized by the fact that a first channel of the fresh water heat exchanger is fed from the high-temperature line via a second circulator pump connected in series, and that it drains into the low-temperature line, and further that the second circulator pump is controlled independently of the first circulator pump. The apparatus according to the invention offers increased security of supply of hot fresh water, even if a first heat source of relatively low power is used. Moreover, the available alternative energy is utilized in the best way possible.

Preheating the fresh water can be achieved in a particularly energy-efficient way by providing inside the storage tank an additional heat exchanger, whose upstream side is connected to a cold-water input fitting and whose downstream side is connected to a second mixing valve, which is in turn connected to a second channel of the fresh water heat exchanger.

A particularly simple design of the apparatus may be achieved by providing a solar collector as a second heat source for heating the heat transport medium, and by configuring the system of solar collector, storage tank, first heat source and, if present, heating system of a building, as a closed pressurized system filled with a single heat transport medium, and by further providing that in addition to the heat transport medium the storage tank be filled with gas.

As a first heat source a condensing gas boiler could for instance be used, but other particularly advantageous possibilities would be Stirling engines, internal combustion engines or fuel cells. The reason for this is that for reasons of cost devices of relatively low power should be used and that usually it is not economical to provide for modulated operation. The present invention is ideally suited for employing such devices.

A further particular advantage of the invention is that the whole apparatus can be designed as an integrated compact device of standard dimensions. For use in an individual household the integrated device can be of size 60×60×200 cm and will cover all fresh water and room heating needs.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail with reference to the enclosed drawing. FIG. 1 shows a schematic circuit layout of a system according to the invention.

DETAILED DESCRIPTION

The system proposed by the invention comprises a solar collector 1 and a storage tank 2. Via a feeder line 4 departing from the bottom region of the storage tank 2 and a feeder pump 5 heat transport medium is fed to the solar collector 1, which medium after having been heated returns via a return line 6 into a stratification tube 7 and thus into the storage tank 2. The stratification tube 7 is provided with a multitude of return-flow openings 7 a positioned in vertical rows, such that temperature stratification prevailing in the storage tank 2 will not be disturbed.

In the upper region 12 of the storage tank 2 there is an air space filled with air pressurized at about 3 bar, with the volume of this space at any rate greater than the total volume of solar collector 1 and the relevant sections of lines 4 and 6 to and from the solar collector 1. When the feeder pump 5 is deactivated the heat transport medium drains back from the solar collector 1 against the feeder-flow direction of the feeder pump 5 into the storage tank 2, the solar collector being positioned at least at a minimum height h above the storage tank 2, and air is sucked into the solar collector 1 from the upper region 12 of the storage tank 2 via the return line 6. The water level 11 in the storage tank 2 will rise and the air space is diminished. The system is however laid out in such a way that a minimum air space will remain in the storage tank 2. Since the solar collector 1 is completely drained the danger of freezing at low temperatures is avoided. On the other hand the solar collector 1 can also be drained if there is a surplus of unused solar heat, thus avoiding the dangers of overheating and stagnation.

The air space 12 in the storage tank 2 simultaneously serves as an expansion space for the room-heating system 3 a, 3 b, which thus is kept at an appropriate pressure level.

Fresh water heating and the room-heating system will now be described. From the storage tank 2 a low-temperature line 26 runs at first to a first circulator pump 22 and through a first heat source 20 into a high-temperature line 25, which opens into the upper region of the storage tank 2. A fresh water heating line 30 branches off from the high-temperature line 25, runs through a first channel 21 a of a fresh water heat exchanger 21 to a second circulator pump 23 and opens into the low-temperature line 26 on the upstream side of the first circulator pump 22. The fresh water heating line 30 contains a first mixing valve 24 from which departs a mixing line 24 a opening into the fresh water heating line 30 downstream of the second circulator pump 23. Via this first mixing valve 24 and the mixing line 24 a a partial stream of the heat transport medium may be cycled by the second circulator pump 23, thus limiting the temperature level in the first channel 21 a of the fresh water heat exchanger 21 and thereby reducing lime deposition.

Departing from the cold-water fitting 31, a cold water preheating line 32 leads to another heat exchanger 10, which is a spiral-coil heat exchanger and is positioned in the bottom region of the storage tank 2. The preheated cold water is fed via a line 33 to a second mixing valve 28, in which the preheated cold water from line 33 is mixed with cold water directly from the fitting 31 as needed to limit the maximum temperature. Subsequently the fresh water is fed through the second channel 21 b of the fresh water heat exchanger 21 and heated to the desired target temperature. Reference number 34 indicates a circulation line which is served by a circulator pump 35.

Furthermore there are provided two room-heating systems 3 a, 3 b, where the first system 3 a runs at higher temperature, for instance a system with radiators. The second system 3 b is a low-temperature system, such as a floor or wall heating system.

The first heating system 3 a is fed from the high-temperature line 25 and has a heating circulator pump 8 a and a heating mixing valve 36 a. Analogously, the second heating system 3 b has a heating circulator pump 8 b and a heating mixing valve 36.

The return flow of the first heating system 3 a finally enters the low-temperature line 26, while the return flow of the second heating system 3 b enters the storage tank 2 below the fitting for the low-temperature line 26 and a stratification baffle 37.

A temperature sensor 27 is placed in the upper fourth of the storage tank 2, which registers the temperature of the heat transport medium at this point. Depending on the measured temperature value the first heat source 20 is switched on or off. A control unit 29 controls primarily the circulator pumps 22 and 23, but also the valves 24, 28, 36 a and 36 b.

Operation of the apparatus according to the invention will now be described.

If the temperature sensor 27 registers a temperature in the storage tank, which is lower than a predefined target temperature, the first heat source 20 is activated to load the storage tank 2. The target temperature at 27 is typically set somewhat higher than the desired fresh water temperature, it is for instance 60° C. if the fresh water target temperature is 55° C. Via the first circulator pump 22 heat transport medium is taken from a region of the storage tank 2 above the stratification baffle 37 and fed back into the upper region of the storage tank 2. If no heat is demanded at the fresh water heat exchanger 21 the total power of the first heat source 20 is used to load the storage tank 2. If there is a small demand for heat at the fresh water heat exchanger 21 a partial stream is branched off the high temperature line 25 by activating the second circulator pump 23, and is used to heat the fresh water. The remaining partial stream continues to load the storage tank 2.

If the heat demand for fresh water exceeds the heat delivered by the first heat source 20, a second operational state is entered in which the second circulator pump 23 is controlled in such a way that its flow rate is greater than that of the first circulator pump 22. Thus the flow direction in the high-temperature line 25 is reversed and hot heat transport medium is sucked from the top of the storage tank 2. It is thus possible to deliver sufficient energy to the fresh water heat exchanger 21.

If the temperature sensor 27 registers a sufficiently high temperature, the circulator pump 22 is not activated and the first heat source is not switched on. The fresh water heat exchanger 21 is now only supplied with heat transport medium from the storage tank 2 via the second circulator pump 23. If the temperature in the high-temperature line 25 is significantly higher than the target temperature of the fresh water, a partial stream of the heat transport medium is cycled by the first mixer valve 24, as has already been described above.

The circulator pumps 8 a, 8 b of the heating system may also be involved in the control of fresh water heating. It is for instance possible to throttle the heating circulator pumps 8 a, 8 b and thus give priority to fresh water heating, if an extremely high fresh water demand has to be met.

As an alternative to the solution shown in FIG. 1, the first heating system 3 a may be fed directly from the storage tank 2 and not from the high-temperature line 25. In this instance it would be of advantage to place a corresponding fitting on the storage tank 2 somewhat below the fitting for the high-temperature line 25, to reserve the topmost region of the storage tank for fresh water heating. This would also give higher priority to fresh water heating.

The solution proposed by the invention will permit the design of a compact integrated system of high reliability and high energy efficiency. 

1. A method for heating fresh water, and for providing heat for heating of buildings, with a first heat source, a storage tank and a fresh water heat exchanger for heating fresh water, the method comprising: conducting a flow of a heat transport medium through the first heat source by a first circulator pump; conducting a flow of the heat transport medium through a first channel of the fresh water heat exchanger by a second circulator pump; wherein the first channel of the fresh water heat exchanger is connected in parallel with the storage tank and a control device is provided, which controls the first circulator pump and the second circulator pump in such a way that in a first operational state a mass flow of the heat transport medium through the first heat source is greater than a mass flow of the heat transport medium through the first channel of the fresh water heat exchanger, and that in a second operational state the mass flow of the heat transport medium through the first heat source is smaller than the mass flow of the heat transport medium through the first channel of the fresh water heat exchanger.
 2. The method according to claim 1, wherein a partial stream of the heat transport medium is cycled by a second circulator pump via a first mixing valve and a mixing line, if a predefined threshold temperature in the fresh water heat exchanger is exceeded.
 3. The method according to claim 1, wherein the fresh water is preheated in a region of the storage tank and is then fed through a second channel of the fresh water heat exchanger.
 4. The method according to claim 1, wherein temperature stratification is maintained in the storage tank, and wherein in the first operational state the heat transport medium heated by the first heat source is fed into an upper region of the storage tank, while in the second operational state the heat transport medium exiting from the first channel of the fresh water heat exchanger is fed into a lower region of the storage tank.
 5. The method according to claim 1, wherein between the first and second operational states a plurality of other operational states can be selected, which are characterized by a different ratio each between the mass flow of the heat transport medium through the first heat source and the mass flow of the heat transport medium through the first channel of the fresh water heat exchanger.
 6. The method according to claim 1, wherein the heat transport medium for fresh water heating is fed directly into a heating system.
 7. The method according to claim 6, wherein the heat transport medium for a first heating system is taken from a high-temperature line, which connects a downstream side of the first heat source to the storage tank, while the heat transport medium for a second heating system is taken from a low-temperature line, which connects an upstream side of the first heat source to the storage tank.
 8. The method according to claim 6, wherein a back flow of at least one heating system is returned into the storage tank at a point below fittings for the fresh water heating system.
 9. The method according to claim 6, wherein the heat transport medium fills and flows through a solar collector when insulation is present to heat the heat transport medium, and wherein the solar collector is drained otherwise, the heat transport medium being collected in a storage tank and kept under increased pressure in the storage tank as well as in the solar collector, and wherein the storage tank is filled partially with the heat transport medium and partially with gas in all operational states.
 10. The method according to claim 9, wherein the heat transport medium present in the storage tank has a free surface.
 11. The method according to claim 1, wherein a pressure in a system comprising the storage tank, the solar collector and the heating system is maintained at a value of between about 2 bar and about 5 bar.
 12. An apparatus for heating fresh water and for providing heat for heating of buildings, comprising: a first heat source, a storage tank and a fresh water heat exchanger for heating fresh water, wherein a downstream side of the first heat source is connected to the storage tank via a high-temperature line and an upstream side of the first heat source is connected to the storage tank via a low-temperature line, said apparatus further comprising a first circulator pump effecting the flow through the first heat source, wherein a first channel of the fresh water heat exchanger is fed from the high-temperature line by a second circulator pump connected in series with the first channel of the fresh water heat exchanger and is drained into the low-temperature line, the second circulator pump being controlled independently of the first circulator pump.
 13. An apparatus according to claim 12, wherein the first channel of the fresh water heat exchanger is connected to a first mixing valve and a mixing line, through which a partial stream of the heat transport medium can be cycled.
 14. An apparatus according to claim 12, wherein a further heat exchanger is provided in the storage tank, whose upstream side is connected to a cold-water fitting and whose downstream side is connected to a second mixing valve, which in turn is connected to a second channel of the fresh water heat exchanger.
 15. An apparatus according to claim 12, further comprising a control device which controls the first circulator pump and the second circulator pump in a first operational state in such a way that a mass flow of the heat transport medium through the first heat source is greater than a mass flow of the heat transport medium through the first channel of the fresh water heat exchanger, and which in a second operational state controls the pumps in such a way that the mass flow of the heat transport medium through the first heat source is smaller than the mass flow of the heat transport medium through the first channel of the fresh water heat exchanger.
 16. An apparatus according to claim 12, further comprising a heating system directly hydraulically connected to the storage tank.
 17. An apparatus according to claim 16, further comprising a first heating system, which is fed from the high-temperature line, and a second heating system, which is fed from the low-temperature line.
 18. An apparatus according to claim 17, wherein the first heating system as well as the second heating system are connected to the storage tank on their downstream side.
 19. An apparatus according to claim 12, wherein a solar collector is disposed as a second heat source for heating the heat transport medium, the system of the solar collector, the storage tank, the first heat source and the heating system being configured as a closed system, which can be filled under pressure with a single heat transport medium, and wherein the storage tank can be filled with gas in addition to the heat transport medium.
 20. An apparatus according to claim 19, wherein the storage tank has a gas space in an upper section, which space is disposed directly above a surface of the heat transport medium.
 21. An apparatus according to claim 19, wherein a feeder line to the solar collector is provided, which departs from a lower region of the storage tank and in which a feeder pump is disposed, and wherein a return line is provided, which opens into an upper region of the storage tank above a maximum filling limit for the heat transport medium.
 22. An apparatus according to claim 12, wherein means for maintaining temperature stratification are disposed in the storage tank.
 23. An apparatus according to claim 22, wherein the means for maintaining temperature stratification is configured as a stratification tube.
 24. An apparatus according to claim 12, wherein the first heat source is configured as a condensing gas boiler.
 25. An apparatus according to claim 12, wherein the first heat source is configured as a Stirling engine, an internal combustion engine or a fuel cell. 